As described in U.S. Pat. No. 6,338,472 to Shimazu et al., humidifiers, like those of the field of the present invention, are typically used to humidify process gases supplied to an anode or a cathode of a Solid Polymer Fuel Cell (SPFC). The process gases comprise a fuel gas provided to the anode and an oxidizing gas provided to the cathode. A solid polymer fuel cell generates electrical energy by electrochemical reactions in which protons generated from a fuel supplied to the anode transfer to the cathode through an electrolyte membrane and react with an oxidizing gas supplied to the cathode to produce water. The humidifier of the present invention, however, is not limited to fuel cells, but is generally applicable to the humidification of gases.
Similarly, a Proton Exchange Membrane Fuel Cell (PEMFC), in contrast to the SPFC device, generally consists of three main components: (1) a porous diffusion catalytic anode, (2) a proton conductive membrane, and (3) a porous diffusion catalytic cathode. The PEMFC converts chemical energy to electrical energy with catalytic electrochemical reactions of hydrogen and oxygen at the anode and cathode, respectively. During the process, the conductivity of the proton exchange membrane plays an important role in the performance of the PEMFC. The membrane conductivity, however, depends on its water content. Usually, high water content gives high conductivity. In fact, fuel and oxidant gases must be humidified to maintain adequate water content in the membrane, therefore requiring a humidifier and a method of humidifying reactant gases.
To operate a fuel cell normally, whether a SPFC or a PEMFC, the membrane must be kept wet. To keep the membrane wet, the process gases are typically humidified by one or more of a variety of techniques. For example, one commonly used technique, referred to herein as a “bubbling-type” humidifier, involves bubbling reactant gases up through a container of heated water so that water molecules are taken up with the reactant gases. An energy source is provided to facilitate the water evaporation into the gas bubbles or gas stream through the container.
In order to make gases highly humidified, the flow rate of gases should be low enough, or the residence time in water should be long enough. Also, the distribution of gases in the liquid water has a large influence on the final humidity of gases. A bubbling-type humidifier should also avoid leaving water droplets to be carried out by the gas stream, even when flow rate is relatively low because there is no typically gas-water separation function in such a humidifier. This method of humidification has the advantage of being very simple and inexpensive. However, typical bubbling-type humidifiers cannot deliver 100% relative humidity to gases and only allow relatively low gas flow rate for a certain cross area of humidifier. Another disadvantage of this method is its uncertainty as to just how much humidity has been imparted to gases as the gases leave the outlet of the humidifier. Many factors, including water temperature, flow rate, gas distribution, inlet gas temperature and humidity, physical structure and condition of the humidifier, affect humidity at outlet.
In bubbling-type humidifiers, it is difficult to control humidity unless using a feedback signal developed from the measurement of actual humidity value at the humidifier outlet. Even with such controls, the bubbling-type humidifier requires a relatively large cross section area and water height to humidify the gas and the response time for change of humidity is very long.
In another conventional method, referred to herein as “steam injection or steam mixing”, water vapor steam is injected, usually in an excess amount, into a dry gas stream to form a gas-vapor-water droplet mixture. The mixture then flows through a heat exchanger to condense down to a set temperature by a chilling coolant. Water droplets and extra water vapor in the mixture condense to a water stream, which is further separated from the gas-vapor stream by a water separator and a water drain. Because there is a condensation process, the set temperature is equal to the Dew Point Temperature. In order to have a condensation procedure, extra water vapor steam must be used. In this technology, a separate boiler, condenser, chiller, water drain and their own individual control systems are usually needed. In order to maintain good control of the dew point temperature, these types of humidifiers are usually bulky, complicated, and expensive with very low energy efficiency.
Another known technique for humidifying reactant gases uses a “membrane-type” humidifier. One example of a membrane-type humidifier is shown and described in U.S. Pat. No. 5,996,976 to Murphy et al. In this technique, water is pumped through a heating element and then directed to one side of a porous membrane. The gases to be humidified are directed across the other side of the membrane. Water molecules penetrate the membrane from the heated water side to the reactant gas side where the water molecules evaporate into the gases and the gases absorb heat from the water. The water may be circulated through a heating chamber as described, or the water may be heated directly in an evaporation chamber. The temperature of the gas-vapor mixture is lower than the temperature of the water because evaporation occurs at the surface of the membrane. Because of this phenomenon, the temperature and humidity of the gas-vapor mixture is rather difficult to control. Further, the difficulty of control increases as the rate of gas flow increases because the amount of heat absorbed from the water is relatively high. Further, a specialized membrane is required, increasing the overall cost of such a system. Again, there is no mechanism to guarantee precise humidity control without further condensation or employment of a humidity sensor.
U.S. Pat. No. 5,262,250 to Watanabe and U.S. Pat. No. 5,952,119 to Wilson teach a kind of self-humidification method for membrane electrode assemblies of fuel cells. The former uses some narrow path or wicks within a membrane and the latter sews hydrophilic thread through a backing layer to enhance the humidification of the membrane. However effective such self-humidification may be in a laboratory environment, it is difficult for commercial manufacturing in a large scale.
Yet another technique for the humidification of a gas involves the application of ultrasonic energy to the gas and a water bath. A quantity of water is contained within an enclosure and gas is introduced to the volume within the enclosure above the surface of the water. An ultrasonic energy source within the enclosure extends through the gas volume into the water bath. Application of ultrasonic energy generates water vapor, which is taken up by the gas and the gas-vapor mixture is withdrawn from the enclosure. This technique has the advantage of easily controllable humidity of the gas-vapor mixture for “batch” processing of gas, but is not suitable to generate and control the humidity of a continuous stream of gas.
Still another technique for humidification of a gas involves a variation of the steam-injection-type humidifier, wherein water is injected onto a hot element, such as a plate, to evaporate the water into an enclosure. Gas is pumped into the enclosure to mix with the water vapor to develop a gas-vapor mixture. The amount of water that is injected onto the heating element is calculated and controlled to meet certain humidity requirements. Further, the temperature of the exit gas-vapor mixture is controlled by controlling the temperature of the heating element.
However, this factor presents a drawback of this technique in that the heating component must use a certain minimum power to reach a temperature sufficiently high to flash the water to vapor instantly and this minimum temperature is usually much higher than the preferred mixture temperature. Also, it is difficult to quickly change the temperature of the heating element when the flow rate of gas or water changes and it is difficult to precisely control the temperature of the gas-vapor mixture, thus the mixture is likely to be overheated. Even if the mixture temperature can be adequately controlled, the range of flow rate and the range of temperature is unacceptably limited using this technique. This is because this technique requires the simultaneous control of two parameters, i.e. the temperature of the gas-vapor mixture and the temperature of the heating element, in one control loop by one means, i.e. the power to the heating element. It is difficult, if not impossible, to simultaneously control these parameters in a realistic control mechanism.
One proposed solution to this control problem involves the use of a condenser in the stream for the gas-vapor mixture. In principle, the humidification is carried out in two steps and two devices. The first step involves steam injection as previously described to generate an over-heated, over-humidified gas-vapor mixture. The second step involves passing the mixture through the condenser to condense the gas-vapor mixture at its dew point. A chiller is required to carry away the heat released from the condensation to maintain the condenser at the dew point. Thus, additional energy is needed to generate the over-heated and over-humidified mixture in the first step, and even more energy is required to drive the chiller to dissipate the additional heat from the cooling and condensation of the mixture. This means that this technique is very energy inefficient, and it is also bulky, complicated, and expensive to build and use.
Another humidification method for PEMFC is taught in U.S. Pat. No. 6,383,671 to Andrews. This method uses a heater to vaporize liquid water and then lets the steam directly mix with dry gases. Effective humidification may obtained for the reactant gases under certain conditions. However, one of problems with this apparatus is that the reactant gases are not pre-heated; that is, the temperature and humidity of gases are far from saturation status of dew point. When vapor mixes with these cool gases, condensation occurs. This phenomenon makes the precise control of humidity difficult because the gases and steam vapor are not mixed evenly. In order to mix the gases and vapor well, the practical size of this humidifier must be quite large.
Thus, there remains a need for a system and a method of humidifying gases that is energy efficient, simple, and easy to control, and more importantly, provides precisely a desired amount of humidification of a continuous gas stream. The present invention is directed to such a solution.